1. Field of the Invention
This invention relates to a method for spheroidizing particles. The method according to this invention is suitable for a wide variety of materials, including those that have a relatively high spheroidization temperature, and/or a relatively large average diameter.
2. Description of Prior Art
Methods for spheroidizing particles can generally be divided into two categories, depending on the type of material to be spheroidized.
The first category relates to methods for spheroidizing material that has an average diameter larger than about 0.5 mm, and has a relatively low softening temperature. Unprocessed glass is one example of such a material. A method for spheroidizing such material generally consists of introducing the raw material into one portion of a furnace and introducing a flame into another portion of the furnace. The combustion products of the flame heat the particles to a softening temperature. The central hottest portion of the flame envelope itself is not directly involved with convective heat transfer to the particles.
The second category relates to methods for spheroidizing material that has an average diameter smaller than about 0.10 mm to 0.20 mm, and includes material having a relatively low softening temperature, such as unprocessed glass, as well as material having a relatively high softening temperature, such as ceramics. Methods for spheroidizing such materials include introducing the material into the hottest portion of a specially-shaped flame. U.S. Pat. No. 4,890,562 teaches a method and apparatus for such inflame treatment of material that utilizes an oxygen-fuel or oxygen-air-fuel flame. Although the apparatus of the '562 patent provides a 4000.degree. F. flame, it does not provide for sufficient retention time of the material in the flame necessary to heat ambient-temperature material that has an average diameter larger than about 0.35 mm to 0.45 mm and a relatively high softening temperature to a temperature sufficient for spheroidization.
Sufficient in-flame retention time to promote spheroidization of particles is only one aspect of such a process. A second, and often equally important aspect of a process for spheroidizing particles is the resultant strength of the spheroidized particles. Thermal stresses in particles during spheroidization occur because of non-uniformity of temperature and structure inversions in the particles. Mechanical properties, including strength, of non-metallic particles such as ceramics, for example, are very sensitive to the physical and chemical properties of their surfaces. When hot particles are rapidly cooled, tension stresses appear near the surface of the particles. If the cooling rate is too rapid, the undesirable structure change and high stresses can reduce the strength of the spheroidized particles. For some particles, such as those used as proppants, it is important that the particles have as high a strength as possible. Conventional methods for spheroidizing particles do not include controlled staged cooling which is required to ensure that the particles can withstand relatively high compressive forces.
U.S. Pat. No. 3,272,615 teaches a method for spheroidizing particles, particularly ferro-silicon particles smaller than 270 mesh. The method of the '615 patent includes imparting a swirling motion to the raw material and passing the swirling raw material through a high-temperature flame. The '615 patent teaches that the swirling motion must be sufficient to maintain the particles in the flame until at least the surfaces of the particles melt.
U.S. Pat. No. 3,015,852 teaches a method for passing ferro-silicon particles through a downwardly directed flame that has a reducing zone at a perimeter of the flame.
U.S. Pat. No. 4,221,554 teaches a method and apparatus for spheroidizing thermoplastic material. The method of the '554 patent includes blowing a stream of thermoplastic particles dispersed in a gas into a jet of pressurized hot gas, and then cooling the spherical particles.
U.S. Pat. No. 5,253,991 teaches a method and apparatus for spheroidizing inorganic material such as cement and slag. The method of the '991 patent includes passing the material through a flame and then rapidly cooling the material.
Thus, in view of the teachings of the related art, it is apparent that there is a need for a method for spheroidizing material that has a relatively large average diameter and/or a relatively high softening temperature. Further, it is apparent that there is a need for a method for producing spheroidized particles which can withstand high compressive forces.